1. Field of the Invention
The present invention relates to a unit for calibrating an output image in image formation applied to an image processing apparatus such as a printer and a copying machine for performing image processing using an electrophotographic method.
2. Description of the Related Art
In an image processing apparatus such as a copying machine and a printer apparatus, a technique for calibrating an output image is conventionally employed. This technique corrects a gamma value provided from an image printed on a sheet of paper having a certain characteristic. Specifically, a gradation pattern is first printed on a specific type of paper at the time of execution of calibration. Then, the printed pattern is read by a reading device, and a result of the reading is used to calculate a gamma correction table. According to the related-art technique, therefore, when the specific type of paper used at the time of execution of calibration is used for printing, calibration can be executed. However, when other types of paper are used for printing, calibration cannot be executed.
Since there are many types of printing paper each of which has a different transferability (an ability to absorb toner into paper by charging) and fixability (an ability to fix toner to paper by heat and pressure), the calibration cannot be executed on the other types of paper. That is, when paper having a different characteristic is used, an output image differs in density. Thus, calibration needs to be executed using paper having the same characteristic as paper to be actually used for image printing. Otherwise, correct calibration cannot be performed. Japanese Patent Application Laid-Open No. 2011-107374 discusses a technique for solving such an issue.
This technique, for example, previously acquires and stores, in the apparatus, a correlation between a certain characteristic (hereinafter referred to as a first characteristic) of reference paper and a characteristic of non-reference paper. The reference paper is used for calibration, and the non-reference paper has a second characteristic different from the first characteristic.
For example, the reference paper having the first characteristic is to be plain paper serving as recommended paper for calibration, and the non-reference paper having the second characteristic is to be thick paper.
According to such a technique, when calibration using the reference paper is executed, a gamma correction table to be used when printing is performed on the non-reference paper can be obtained based on a gamma correction table generated by the calibration and the paper characteristic correlation.
Therefore, whenever calibration is executed using the reference paper, the gamma correction table is automatically updated using the paper characteristic correlation without executing calibration using the non-reference paper.
The execution of the related-art technique requires reference paper of which a sheet characteristic is well defined. However, an operation unit of the apparatus for performing calibration displays characteristics roughly classified by paper type on a display unit for the sake of user convenience when a paper type indicating a type of paper is displayed on the display unit. Accordingly, the detailed characteristic of the reference paper cannot be displayed, and the characteristic of the paper to be used for printing is displayed on a roughly-classified paper type basis on the display unit.
For example, paper having a grammage of approximately 65 g/m2 to 90 g/m2 is classified as plain paper. That is, paper (reference paper) having a first characteristic with a grammage of 81 g/m2 and paper having a third characteristic with a grammage of 68 g/m2 are classified as plain paper of the same paper type.
Thus, although each paper actually has a different characteristic, both of the paper are classified as plain paper. Consequently, a user has difficulty in determining whether a characteristic of paper to be used for calibration is the same as that of the reference paper (the first characteristic).
If paper used by the user as the reference paper is the third paper, a gamma correction table, different from that obtained when calibration is performed using the first paper, is obtained.
In such a case, a value serving as a reference differs, causing inaccuracy of the characteristic correlation between the reference paper and the non-reference paper. Consequently, for example, a gamma correction table for non-reference paper such as thick paper becomes inaccurate.